1. Field of the Invention
The present invention relates to a method of manufacturing a substrate for an organic electroluminescent display device, which comprises an optical element such as a diffraction grating, etc.
2. Description of the Related Art
An organic electroluminescent device (hereinafter, referred to as “organic EL device”) is an emitting device employing a principle that fluorescent material emits light using recombination energy of holes injected from an anode and an electrons injected from a cathode. As an exemplary organic EL device, there is a low-voltage drive organic EL device with layered devices, reported by Tang C. W. et al. (Tang, C. W., VanSlyke, S. A. Applied Physics Letters, Vol. 51, p 913, 1987), and such layered device has remarkably improved luminescence properties of the organic EL device. Further, the organic EL device has been being widely developed for recent practical utilization since such a high powered organic EL device was developed.
The organic EL device having a two-layered structure, developed by Tang et al., comprises an emitting layer containing tris (8-quinolinol) aluminum (AlQ), and a hole transporting layer containing a tri-phenyl thiamine derivative such as tri-phenyl thiamine diphosphate (TDP). The reason why the two-layered structure gives good luminescence properties is because efficiency of injecting the holes into the emitting layer is increased, efficiency of creating excitons by means of the recombination is increased by blocking the electrons injected from the cathode, and the created excitons can be encapsulated in the emitting layer. Also, as an exemplary advanced structure of the two-layered structure, there has been reported a three-layered structure comprising a hole transporting layer, an emitting layer, and an electron transporting layer. Together with the two-layered structure composed of the hole transporting layer and the electron transportable emitting layer, this three-layered structure has been known as a representative structure of the organic EL device. Further, such a layered device is being required to improve its recombination efficiency of the holes and the electrons. For this purpose, various studies have been performed.
Because the organic EL device is a spontaneous emitting device having high-speed response characteristic, it is expected that the organic EL device is practically used as a fine pitch display for portable terminals and televisions. However, to produce the fine pitch organic EL display on a commercial scale, the organic EL device should be improved in luminous efficiency. Therefore, necessity to improve the luminous efficiency will be described hereinbelow.
First, in consideration of a carrier recombination principle of the organic EL device, the electrons and the holes, which are injected from the electrodes respectively, are formed into electron-hole pairs by a coulomb reaction, wherein some of the electron-hole pairs are singlet-excitons and some of them are triplet-excitons. Here, the singlet-excitons and the triplet-excitons are created at a ratio of 1 to 3. That is, when there is no phosphorescence in a triplet state, the maximum quantum yield of the emission is 25%, which means that the maximum efficiency in the organic EL device is 25%. Further, in the organic EL device, when a refraction angle of light is larger than a critical angle depending on a refractive index of the emitting layer, the light is totally reflected and thus cannot escape.
That is, if the emitting layer has a refractive index of 1.6, effective emission is about 20% of the whole emission. Additionally, under the consideration of the creation ratio of the singlet-excitons, the effective emission is about 5% of the whole emission. Thus, the luminous efficiency of the organic EL device is lowered (Tsutsuidetsuo, “Effects and trends in organic electroluminescence,” Display Monthly, Vol. 1, No. 3, p 11, September 1995). For this reason, there is no alternative but to improve the luminous efficiency of the organic EL device.
To improve the luminous efficiency, there has proposed various methods of developing inorganic EL device technology, etc. By way of examples of the methods, there have been disclosed a condensed structure of a substrate (Japanese Patent Publication No. S63 (1988)-314795) and a reflection surface formed in a side of the device (Japanese Patent Publication No. H01(1989)-220394).
The foregoing examples are effective in a relatively large-sized substrate, but are disadvantageous in the fine pitch display having a small-sized substrate because it is difficult to manufacture a condensed lens and to form the reflection surface onto the side of the device. Further, in the organic EL device comprises the emitting layer having a thickness of several microns or less, it is very difficult for an ultra fine processing technology to form a reflector on the side of the organic EL device. Also, even if the reflector can be formed on the side of the organic EL device, a production cost thereof is so high that it is disadvantageous in the practical utilization.
As another example, there is disclosed a structure employing a flat layer interposed between a glass substrate and an emitting layer as an anti-reflection layer, wherein the refractive index of the flat layer is an intermediate value of the respective refractive indexes of the glass substrate and the emitting layer (Japanese Patent Publication No. S62(1987)-172691).
However, this structure improves the luminous efficiency limited to a frontward direction, but it is impossible to prevent a total reflection. That is, this anti-reflection layer is effective in the emitting layer having a high refractive index such as the inorganic EL device, but it is difficult to improve the luminous efficiency in the organic EL device which has a low refractive index as compared with the inorganic EL device.
As described above, there have been proposed various structures to improve the luminous efficiency of the organic EL device, but they are not enough to satisfy desirable performance. Accordingly, there has been disclosed still another structure comprising a substrate formed with an optical element such as a diffraction grating (Japanese Patent Publication No. H11(1999)-283751), which is considered to be effective to improve the luminous efficiency.
The above-described structures disclosed in Japanese Patent Publication Nos. 1988-314795 and 1989-220394 are effective in a relatively large-sized substrate, but is disadvantageous in the fine pitch display having a small-sized substrate because it is difficult to manufacture a condensed lens and to form the reflection surface onto the side of the device. Further, in the organic EL device where the emitting layer has a thickness of several microns or less, it is very difficult to form a reflector on the side of the organic EL device by use of an ultra fine processing technology. Also, even if the reflector can be formed on the side of the organic EL device, a production cost thereof is too high to be disadvantageous in the practical utilization.
Further, the structure disclosed in Japanese Patent Publication No. 1987-172691 improves the luminous efficiency in one (e.g. frontward) direction, but it is impossible to prevent a total reflection.
That is, this anti-reflection layer is effective in the emitting layer having a high refractive index such as the inorganic EL device, but it is difficult to improve the luminous efficiency in the organic EL device which has a low refractive index as compared with the inorganic EL device.
As described above, there have been proposed various structures to improve the luminous efficiency of the organic EL device, but they are not enough to satisfy desirable performance. Accordingly, there has been disclosed the structure disclosed in Japanese Patent Publication No. 1999-283751, and a substrate formed with an optical element such as a diffraction grating is considered to be effective to improve the luminous efficiency of the organic EL device.
However, in the organic EL device comprising the optical element, disclosed in Japanese Patent Publication No. 1999-283751, the luminous efficiency can be remarkable enhanced, but it is difficult to manufacture this structure. Particularly, it is difficult to stably form a good optical element, and variation is generated between luminescence properties in the surface of the substrate having the optical element.
Additionally, a state-of-the-art processing technology is inevitable to form grooves of the diffraction grating as the optical element by a fine pitch, and a production cost is highly increased. Therefore, there is required technology for stably and easily forming the optical element on the substrate.
Further, it is very difficult to fill the grooves during the process of forming the optical element. Typically, layer formation technology such as a sputtering method is used for filling the grooves of the optical element. However, in the conventional layer formation technology, it is impossible to completely and uniformly fill the grooves with the layer because the layer is formed depending on a surface profile of the substrate.